Although an R-T-B (“R” represents one or more of rare earth elements and “T” represents one or more of transition metal elements including Fe or Fe and Co) based sintered magnet shows excellent magnetic properties, it tends to show a low corrosion resistance due to inclusion of easily oxidized rare earth elements as its main component.
Thus, to improve corrosion resistance of the R-T-B based sintered magnet, a magnetic body is generally subject to surface treatment, such as resin coating and plating. On the other hand, addition elements or internal structures of a magnetic body have been changing to improve corrosion resistance of the magnetic body itself. Improvement in corrosion resistance of a magnetic body itself is extremely important to enhance reliability of surface-treated products. This improvement allows products to be applied by surface treatment that is easier than resin coating or plating, and contributes to cost reduction of products.
For example, Patent Document 1 conventionally suggests a technique to improve corrosion resistance of a magnet by reducing carbon content in a permanent-magnet alloy to 0.04 mass % or less and controlling intermetallics R—C of rare earth element and carbon in a non-magnetic R-rich phase to 1.0 mass % or less. In addition, Patent Document 2 conventionally suggests a technique to improve corrosion resistance by setting Co concentration in an R-rich phase to 5 to 12 mass %.
However, in conventionally used R-T-B based sintered magnets, water such as water vapor under a use environment oxidizes “R” in the R-T-B based sintered magnet and generates hydrogen. Then, an R-rich phase in a grain boundary absorbs said hydrogen. As a result, corrosion of the R-rich phase progresses, and magnetic properties of the R-T-B based sintered magnet deteriorate.
An additive amount of lubricant, which is added for improving magnetic field orientation when pressing in a magnetic field, needs to greatly be decreased for reducing carbon content in a magnet alloy to 0.04 mass % or less, as Patent Document 1 suggests. Thus, degree of orientation of magnetic powder in a green compact decreases, and residual magnetic flux density Br after sintering decreases. As a result, a magnet having sufficient magnetic properties cannot be obtained.
Co additive amount in a raw material composition needs to rise for increasing Co concentration in an R-rich phase, as Patent Document 2 suggests. However, since Co also enters in a main phase of R2T14B phase by replacing Fe, it is unable to increase Co concentration only in the R-rich phase and thus necessary to add Co beyond that required in the R-rich phase. Thus, production cost rises due to increase of using amount of expensive Co, and magnetic properties deteriorate due to substitution of Co for Fe in the main phase with more than necessary.